Saltmarsh Vegetation Zone Determines Microbial Nitrate Reduction Responses to Elevated Nitrate and Organic Matter Inputs

Abstract Anthropogenic nitrogen (N) has more than doubled over the last century, with much of this excess N making its way to the coastal zone. Coastal marshes remove excess N via denitrification or, conversely, retain and recycle N through dissimilatory nitrate reduction to ammonium (DNRA). Organic carbon (C) and N availability are important for determining the partitioning of these two competing N-reduction pathways. Vegetation inherently links the C and N cycles by uptaking and releasing both elements. However, sea level rise and eutrophication are altering marsh vegetation composition, and subsequently, sediment microbial communities with direct consequences for ecosystem functions, such as N-removal. We tested how organic C and N additions affect partitioning of N-removal and N-retention processes among sediments from different vegetation zones in a salt marsh while controlling hydro-edaphic variables. We found sediments from Distichlis spicata patches had the lowest capacity for N-removal while sediments form Juncus roemerianus and Cladium mariscus patches had higher N-removal capacity. Under enhanced soil organic C and excess N loading, NO3- reduction responded differently depending on vegetation zone. N-retention increased 3-fold in the mid-marsh patches (Juncus and Distichlis) and dominated NO3- reduction processes in response to high C and N inputs, while N-retention and N-removal were offset in the high marsh Cladium patches. Our findings help to forecast how shifts in vegetation community following dynamic climatic conditions may affect global N processing in areas experiencing excess N loading.